The principle of recirculating a portion of the exhaust gases of a combustion engine at the intake is known, for example from patent U.S. Pat. No. 8,862,369. When applied to a supercharged spark-ignition engine, this technology can improve the efficiency of the engine, thereby reducing fuel consumption. Exhaust gas recirculation is commonly referred to using the abbreviation EGR. The EGR flow rate and the EGR ratio used by the engine are discussed below. In order to optimize operation of the engine, the quantity of exhaust gases recirculated is adapted to the operating conditions and is precisely controlled by an electric valve controlled by the electronic control unit of the engine of the vehicle. In steady-speed operation, the flow rate of recirculated exhaust gases can be approximately 20% of the total flow rate. Conversely, during idle phases, the EGR flow rate is usually zero, since the presence of EGR adversely affects combustion quality. The exhaust gases are usually recirculated upstream of the metering throttle of the combustion air admitted to the engine. During operating phases with a high EGR ratio, the quantity of exhaust gas between the exhaust gas recirculation valve and the gas throttle can be high.
When the driver stops accelerating by completely lifting his foot off the accelerator, the metering throttle closes almost completely in order to reduce the engine torque produced and to achieve engine braking. The recirculated exhaust gases in the circuit portion located downstream of the exhaust gas recirculation valve and upstream of the gas throttle can only be evacuated by being consumed by the engine. Since the quantity of air admitted to the engine is then very low, these recirculated exhaust gases are only evacuated very slowly. Consequently, a very large amount of recirculated exhaust gases may remain in the intake circuit when the engine enters idle control, even if the exhaust gas recirculation valve is completely closed. Under these operating conditions, combustion stability can be highly degraded by the presence of these unwanted exhaust gases.
These combustion instabilities can generate uncomfortable vibrations for the occupants of the vehicle and increase the quantity of pollutants emitted. The purpose of the invention is to overcome these drawbacks.